Thursday, February 28, 2013

Screech Owls and Blindsnakes: An Unlikely Mutualism

In the 1970s and 80s, a pair of biologists at Baylor University in Waco, Texas, Fred Gehlbach and Robert Baldridge, were studying screech owl nesting ecology. These small owls nest in tree cavities and eat a variety of small animals, from insects to mice. Like most raptorial birds, Eastern Screech Owls usually kill their prey before bringing it home to feed to their nestlings. Gehlbach and Baldridge observed some of the screech owls in their study carrying live Texas Blindsnakes (Rena [formerly Leptotyphlops] dulcis) to their nests in experimental nest boxes like those used by wood ducks and bluebirds (pictured at right). When they checked the nests the next day, they found, to their surprise, between one and fifteen live blindsnakes living among the owl chicks in fourteen different nests! In some cases, the snakes lived with the baby owls for at least a week! Many of the blindsnakes bore scars from adult owl beaks, but few had been killed.

If you're not familiar with blindsnakes (aka scolecophidians), don't worry; few people are. There are about 400 species of these 'seriously strange serpents', as Darren Naish calls them over at TetZoo, distributed chiefly in the world's tropical regions (the Texas Blindsnake is one of the few temperate exceptions). Most have small eyes (or none at all, as their name suggests), smooth round scales, and eat invertebrates. Their jaw architecture is entirely unique: their jaws act like little scoops to effectively shovel ant and termite larvae and pupae into their mouths. Check out the video from BBC's Life in Cold Blood below, or visit the homepage of blindsnake biologist Nate Kley at Stony Brook University.

Almost as cute as baby snakes

How does this help baby screech owls? Gehlbach and Baldridge wanted to find out, so they measured the diversity and abundance of invertebrates in the owl nests with and without live blindsnakes, as well as the health and survival of the baby owls (which they were already measuring). They found that nests with blindsnakes had significantly fewer mites, insects, and arachnids, and that baby owls from these nests were 25% more likely to survive and grew as much as 50% faster; in other words, the presence of the blindsnakes improved the health of the baby owls and the fitness of the adults. The effects were more pronounced for the youngest owl babies, which hatch as many as six days later than their oldest sibling. As the nail in the coffin, Gehlbach and Baldridge tested whether or not the blindsnakes actually ate the invertebrates they found in the owl nests, and sure enough, they chowed down on the soft-bodied fly larvae that kill baby owls in nearly 30% of nests.

Texas Blindsnake (Rena dulcis)

They also noticed that blindsnakes were more likely to be found in nests after it rained, probably because the mother owls had an easier time of finding the blindsnakes when they were crawling around on the surface, which many fossorial snakes tend to do when rainwater fills their burrows. Gehlbach and Baldridge also found that blindsnakes could only survive about two weeks in owl nest boxes that did not contain baby owls, suggesting that they were dependent on insect larvae that entered the nest inside food brought by the mother owl. These snakes can climb trees, so presumably it isn't too challenging for them to climb down out of a nest box after it is vacated by owls; one gravid female blindsnake was found in a nest box, so it is possible that they lay their eggs there before leaving. Some nests contained dead blindsnakes, which Gehlbach and Baldridge hypothesized had been eaten by the baby owls after their food supply had run out. In feeding experiments, baby screech owls readily consumed dead blindsnakes as well as other snakes of similar size, such as Rough Earthsnakes (Virginia striatula).

Skull architecture of Rena dulcis

The skulls of blindsnakes are just amazing, and it's thanks to the research efforts of blindsnake anatomist Nate Kley of Stony Brook University that we know so much about them. Kley has characterized the feeding behavior of two families of blindsnakes, the Leptotyphlopidae, which use scooping motions of the lower jaw known as mandibular raking, and the Typhlopidae, which use similar motions of the upper jaws, called maxillary raking. It's remarkable how similar the two strategies are given that the snakes are using entirely different parts of their bodies to employ them and that they are separated by about 110 million years of evolution. High-resolution CT scans of the skill of Rena dulcis are also available from the good people at UT Austin's DigiMorph project. The jaws (upper in typhlopids, lower in leptotyphlopids) move about independently of the skull to a remarkable degree. You can get a really good idea of that motion by watching videos of leptotyphlopids here, here, and here, and of typhlopids here and here. As soon as they're in the mouth, those larvae are goners! These snakes are unlike all others in that they eat huge numbers of prey items very quickly, thanks to their unique jaw architecture. One Blackish Blindsnake (Austrotyphlops nigrescens) from Australia was recorded to have eaten over 1,431 ant larvae/pupae in one sitting! Some blindsnakes have cloacal secretions that aid in repelling attacking ants or chemically camouflaging the blindsnakes, which live inside ant mounds. The list of amazing attributes goes on and on - and there is much more for scientists to find out!

ACKNOWLEDGMENTS

Thanks to Count_Strad, Toby Hibbits, Gary Nafis, and Nate Kley for use of their photos and figures.

Every once in a great while I am thankful for blogging. This is one of those times.

Great post. I had never heard of this. It's not clear to me or to the the authors that the owls are intentionally preserving the snakes for this purpose. It would be awesome to do more work on this and find out if 1. the owls are catching the snakes as prey or for the mutualism, and 2. if similar behavior occurs with other species--for instance other owls and blindsnakes in the tropics.

Andy, I completely agree - and surprisingly, this study has not been followed up that I could find. It seems like ideal raw material for the evolution of a true mutualism, doesn't it? One telling detail that the authors placed in their paper was that most snakes caught by adult owls are killed before transporting them back to the nest, yet the authors observed adult owls flying with live blindsnakes in their bills.

Many tropical blindsnakes seem to be at least partially arboreal, and there is so little known about them that I'd be willing to bet there are other such interactions, but what they are is anyone's guess.

Testing the first question would be a challenge. But the fitness consequences for the owls are pretty large so you would predict that this behavior could be selected for. One idea that comes to mind (perhaps infeasible) is to transplant screech-owls from other populations outside of the range of blindsnakes to this part of texas to see if they do the same thing. If not it would be stronger evidence that the Eastern Screech-Owls in this situation are intentionally doing this. Also you could track individuals and see if offspring raised by parents who do this also do this kind of behavior. Lots of fun long-term project ideas. This is the kind of story that used to most fascinate me when I was a little kid.

Definitely, me too. Great ideas. Since these observations were made 30 years ago, it would be of some interest even to go back to these sites and see if the same interaction is still taking place there today.

Many years ago I was a field tech on a bird nest box project in central Venezuela (green-rumped parrotlets), and we fairly regularly found small snakes in the nest boxes. I think they were a Leptodiera (cat-eyed snake). I never got a sense that the birds objected, but it also did not seem frequent enough for me to suspect the parrotlets were encouraging the process.

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About Me

I am a PhD student at Utah State University, where I study the behavior, physiology, and ecology of lizards and snakes. The many fascinating aspects of snake natural history have led me to research this topic, which is quite narrow compared to my interest in snake ecology as a whole. Additionally, my work brings me into frequent contact with the serious need for snake conservation, which really requires holistic conservation of ecosystem structure and function, on which human society depends. I believe that we can only accomplish this goal through education, and that is partly why I decided to publish this blog. The title is a quote by David Quammen, one of the best science writers around, and the Mudsnake in the logo is from Duméril, Bibron, & Duméril's nine-volume early 19th century opus, Erpétologie Générale.